This invention relates generally to control of liquid heaters such as water heaters. More particularly, this invention pertains to devices and methods for sensing and measuring operating values of flow-through water heaters, as well as using such values to actuate certain control aspects of the water heaters.
Safe operation of water heaters and the like requires that overheating be avoided. In water heaters in which energy is introduced through electrical elements, the water level in the vessel may perchance be below the level of an element. Without contact with the liquid "heat sink", the element may heat to e.g. 1000 degrees F. in less than a minute. The element may melt and fall into the vessel bottom, damaging the coating on a tank or the tank itself. When the vessel wall is constructed of a non-metallic material, heat generated by the overheated element may permanently damage the vessel wall. It may be necessary to shut off the power supply to the element in a fraction of a minute, for example, in order to avoid damage to the element or the vessel.
In the prior art, a thermostat controls the heating elements based on the measured water temperature. In the "dry-fire" condition, the water temperature being sensed at the thermostat may not rise beyond the parameters seen under a normal heating condition, before damage will occur. Thus, reliance cannot be placed on the normal thermostatic control to detect and react to a "dry-fire" state.
The use of thermal fuses in the element port requires that a new fuse be installed upon each thermal overload incident. More importantly, the critical element temperature may sometimes be reached before the fuse overload temperature is reached; the fuse may not respond in time to prevent damage to the elements or water heater vessel.
For gas-fired water heaters, other problems exist. In a modem water heater having a non-metallic vessel, the water is pumped from the vessel through an exterior heating circuit. The circuit includes a heat exchanger, typically a coil, where heat is transferred from the flame and hot combustion gases to the circulating water. A temperature sensor is normally positioned in the heating circuit to measure the water temperature and control the firing cycle.
If the water level in the vessel should drop to below the inlet pipe to the exterior heating circuit, the coil will become empty and the flame and hot combustion gases will overheat the empty coil. The danger of damaging the heat exchanger coil exists.
Other operating disfunctions may also occur. For example, a thermal sensor may be accidently left disconnected from the vessel or the controller. In such cases, the controller will not sense a temperature change, even though the water is being heated. The burner or electrical element will continue to provide thermal energy to the unit even though the maximum planned temperature is exceeded. An unsafe overheat condition may result which damages heater components.
Furthermore, while all water heaters have a safety valve actuated by excess pressure/temperature, this valve will not respond to a dry-fire condition, where there may be no water to expand and overpressurize the vessel, and where any existing water is being heated at a sub-normal rate.
In prior art water heaters, there has been no satisfactory system for minimizing power or fuel usage while simultaneously ensuring adequate hot water supply at the desired temperature and in addition, ensuring safe operation.
The industry needs apparatus and procedures for sensing and calculating actual operating conditions, for detecting unsafe operations resulting from low water levels or disconnected sensors, for taking action to alleviate the unsafe operations, and for determining energy usage and projected heating times.